This invention relates to a process for cleaning the metal surfaces of organically contaminated heat transfer equipment in the petroleum and petrochemical industries to quickly, safely, and economically.
The manufacture of chemicals and petroleum products in the field of this invention consumes enormous amounts of energy. One major refiner—Exxon Mobil—estimates that it expends $190 million dollars in energy per month to operate its refineries and chemical facilities. See The Lamp, Exxon Mobil, Winter 2002. Exxon Mobil production constitutes approximately 10.6% of the United States production capability. Accordingly, one would estimate that more than $1.7 billion dollars of energy is consumed per month in producing these organic products in the petroleum refining industry.
Much of this consumption is due solely to the fouling of system components. The petroleum products and chemicals produced in this field naturally tend to deposit on contact surfaces, causing the equipment to operate sub-optimally. This tendency to deposit exacerbates an already difficult situation. As an example, in an article published in Chemical Engineering Progress, a heat exchanger fouling rate of 0.35 yr-1 was used which when applied to a particular piece of equipment may cause an annual efficiency penalty of 30%. O'Donnell, Barna, Gosling, Chemical Engineering Progress, June 2001. These figures are consistent with the values published by the Tubular Exchanger Manufacturers Association (TEMA) for exchanger fouling resistance. Considering this 30% penalty, if petroleum refining and chemical processing equipment is not cleaned periodically, the resulting cost caused by energy losses attributable to fouling could exceed $500 million. FIG. 1 illustrates how fouling (the result of contaminate deposition on exchanger tube walls) affects the heat exchange coefficient for an exchanger over time. As the heat transfer coefficient decays, more energy must be consumed to accomplish the same fluid heating through the exchanger.
Industry has recognized this problem. An article by O'Donnell, Barna and Gosling describes a method used to compute an optimal cleaning cycle. Industry benchmarks such as the “Solomon Index” rate companies on their ability to optimize their processes. All companies have established an energy reduction and process optimization program. However, prior to this invention, no realistic alternative was available for cleaning heat exchange equipment without stopping the process for a substantial amount of time, subjecting the equipment to metal deteriorating chemistry and deleterious thermal cycles. For example, petroleum refiners use crude preheat exchangers to increase the temperature of crude oil entering distillation towers. These exchangers operate serially with the tower so that if the exchangers are removed from service, the crude feed stops, shutting down the facility. Depending on the nature of the crude, condition of associated equipment, operating temperatures and flow rate, exchangers can foul at a rate of approximately 0.35 Btu/hr Fft2 per year. Typically, refiners will continue to operate these exchangers—despite a 30% annual reduction in efficiency—until the plant is shut down for major maintenance because the cost to shut down the facility and clean the exchangers is too great. Using prior art procedures, exchangers would be removed from service for 3 to 5 days for cleaning. During the prior art procedures, exchangers are subjected to corrosive chemicals, abrasive procedures and large thermal excursions, all of which may damage the equipment or make it impossible to reassemble. Five days of crude unit shutdown may cause a facility to irreversibly lose more than $10 million in revenue. Historically, using prior art practices, this loss in revenue was more costly than the savings provided from cleaning. Thus, a decision was generally made to continue to operate the fouled, inefficient exchangers until efficiency drops so low as to make cleaning cost-effective. If the refinery were able to clean the exchangers more quickly, this decision would be reversed and a great amount of money saved. Before the present invention, however, this was not a possibility.
Other problems with the prior art systems are environmental in nature. The inefficiency caused by fouling causes the emissions of carbon dioxide, sulfur dioxide, nitrogen oxide and other gases to be increased. Thus, a cleaning regimen that improves efficiency also serves to reduce the amount of noxious emissions. The prior art methods also produce large quantities of hazardous waste. These methods typically use water circulation procedures where vessels are completely filled with water and cleaning chemistry. After cleaning, the water tainted with dangerous impurities must be specially treated. A typical refinery turnaround using this kind of water-circulation cleaning procedure will produce approximately 500,000 gallons of hazardous material that must be disposed of at high cost to the refinery while creating a potential ecological nuisance. Likewise, another prior art procedure of blasting solid contaminant from the exchanger using high pressure water also produces large quantities of solid hazardous waste that must be specially treated.
The present invention overcomes these disadvantages in the prior art methods by injecting a cleaning agent into high-pressure steam, and then introducing the steam and cleaning agent, which includes terpenes, into a vented exchanger. Terpenes have been used in refineries before. A liquid-steam method using terpenes is disclosed in U.S. Pat. No. 5,356,482 (“the '482”). The methods disclosed in the '482, however, are much different than those here. The '482 discloses the use of terpenes for removing dangerous and explosive gases from refinery vessels—not for cleaning the metal surfaces inside the exchanger for the purpose of improving heat transfer properties—as with the present invention. The '482 methods are also different in that they involve either the circulation of condensed fluid, or the injection of cleaner into a water circulation. These methods further require the vessel to be sealed under pressure and to cool—a technique that has been known to occasionally cause catastrophic collapse. Unlike the '482 methods, rinsing condensation is not required. Thus, there is no need to reduce the temperature of the vessel to create the necessary condensation. Further, the present invention does not use a microemulsion of cleaning chemical, or rely on mechanical rinsing. Rather, the present invention uses a fully concentrated solution of chemical agent in the vapor form to accomplish the cleaning. Another important difference is that the process of the present invention occurs in a fully vented exchanger. This eliminates any possibility of catastrophic collapse.